usb: dwc3: gadget: Initialize the TRB ring

Clears out all the TRBs in the ring to clean up any stale data that
might be in them from the previous time the endpoint was enabled.

Also removed the existing clear of the LINK trb since the entire ring is
cleard just before.

Signed-off-by: John Youn <johnyoun@synopsys.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>